1. Field of the invention
The present invention relates to a conductive compound and a conductive cross-linked product, both containing a π conjugated conductive polymer. The present invention further relates to capacitors such as aluminum, tantalum, and niobium electrolytic capacitors and a production method thereof. The present invention further relates to an antistatic coating material for imparting antistatic properties to films, an antistatic coating having antistatic properties, an antistatic film used for wrapping food products and electronic parts, an optical filter used for the front surface of liquid crystal displays and plasma displays, and an optical information recording medium such as CDs and DVDs.
2. Description of Related Art
Generally, π conjugated conductive polymers composed of the main chain of a conjugated system containing π electrons are synthesized by electrolytic polymerization or chemical oxidative polymerization.
In the electrolytic polymerization, a previously prepared base such as an electrode material is immersed in a mixed solution of an electrolyte as a dopant and precursor monomers for constituting a π conjugated conductive polymer to form a film of π conjugated conductive polymer on the base. Therefore, mass production is very difficult.
On the other hand, there are no such limitations on the chemical oxidative polymerization. A large amount of a π conjugated conductive polymer can be produced in a solution by adding oxidant and oxidation polymerization catalysis to precursor monomers of the π conjugated conductive polymer.
However, the π conjugated conductive polymer is obtained as an insoluble solid powder in the chemical oxidative polymerization because the polymer becomes less soluble in a solvent as the conjugated system of the main chain of the polymer grows. It is difficult to form a uniform film of a π conjugated conductive polymer on a base surface if the polymer is insoluble.
Therefore, some methods to solubilize the π conjugated conductive polymers have been attempted. They are a method of introducing a functional group into the polymers, a method of dispersing the polymers in a binder resin, and a method of adding an anion group-containing polymeric acid to the polymer.
For example, a method of preparing an aqueous solution of poly(3,4-dialkoxythiophene) by chemical oxidative polymerization of 3,4-dialkoxythiophene using oxidant in the presence of polystyrene sulfonic acid, which is an anion group-containing polymeric acid having a molecular weight of 2000 to 500000, in order to improve the dispersibility in water, is disclosed in Japanese Patent Publication No. 2636968. A method of preparing an aqueous colloid solution of a π conjugated conductive polymer by chemical oxidative polymerization of a precursor monomer of the polymer in the presence of polyacrylic acid, is disclosed in Japanese Unexamined Patent Application, First Publication No. 7-165892.
According to methods disclosed in Japanese Patent Publication No. 2636968 and Japanese Unexamined Patent Application, First Publication No. 7-165892, an aqueous dispersion solution containing a π conjugated conductive polymer can be easily prepared. These methods require a π conjugated conductive polymer to contain a large amount of anion group-containing polymeric acid for ensuring its dispersibility. Therefore, the problem occurs that the obtained conductive compositions contain a large amount of compounds which do not contribute to conductivity, making it difficult to achieve high conductivity.
In the chemical oxidative polymerization, high oxidative oxidants cause unfavorable side reactions in high probability during chemical oxidative polymerization. Therefore, the polymer structures having poor conjugated property may be produced, or the produced polymer may be attacked by the oxidant once again and excessively oxidized, and then the obtained π conjugated conductive polymer has low conductivity. Some methods are used to solve these problems, such as using transition metal ions as catalysis or allowing reaction at a low temperature for a long time. However, these methods fail to sufficiently prevent conductivity from dropping because the produced polymer is attacked by protons produced by dehydrogenation of reactive monomers, and, therefore, the π conjugated conductive polymer may have low structural regularity.
Furthermore, when the conductive composition contains a binder resin, a π conjugated conductive polymer obtained by the chemical oxidative polymerization may have low compatibility with the binder resin.
As an example, π conjugated conductive polymers are used in capacitors.
Along with recent digitalized electronic devices, capacitors used in those electronic devices are required to reduce impedance in high frequency range. In order to meet this requirement, conventionally, a capacitor comprises a dielectric which is an oxide film of valve metals such as aluminum, tantalum, and niobium, and a cathode comprising a π conjugated conductive polymer formed on the surface of the oxide film.
A capacitor generally comprises an anode consisting of a porous material of valve metal, a dielectric layer formed by oxidizing the surface of the anode, and a cathode formed by laminating a solid electrolyte layer, a carbon layer, and a silver layer on the dielectric layer, as shown in Japanese Unexamined Patent Application, First Publication No. 2003-37024. The solid electrolyte layer of a capacitor is composed of a π conjugated conductive polymer such as pyrrole or thiophene. The solid electrolyte layer penetrates into the porous material, and the electrolyte is in contact with the dielectric layer in a larger area for higher electrostatic capacity and restores defective parts of the dielectric layer for preventing current leakage.
Known methods of forming a π conjugated conductive polymer include electrolytic polymerization (Japanese Unexamined Patent Application, First Publication No. 63-158829) and chemical oxidative polymerization (Japanese Unexamined Patent Application, First Publication No. 63-173313).
In electrolytic polymerization, a conductive layer of manganese oxide must be previously formed on the porous material surface of the valve metal. This method is complicated and troublesome, and further manganese oxide has low conductivity and then the effect of using a high conductive π conjugated conductive polymer is impaired.
The chemical oxidative polymerization requires a long polymerization time. It also requires repeated polymerization to ensure thickness. Therefore, the capacitor suffers from low production efficiency and low conductivity.
Furthermore, a method that eliminates the step of forming a conductive polymer on a dielectric layer in the electrolytic polymerization or chemical oxidative polymerization is proposed in Japanese Unexamined Patent Application, First Publication No. 7-105718. Japanese Unexamined Patent Application, First Publication No. 7-105718 describes a method in which aniline is polymerized in the presence of a polymeric acid containing a sulfo group and carboxy group to prepare water-soluble polyaniline and the polyaniline aqueous solution is applied to the dielectric layer and dried. This method is simple. However, the polyaniline solution does not sufficiently penetrate into the porous material and a polymeric acid used along with a π conjugated conductive polymer leads to low conductivity, which, in some cases, may be temperature dependent because of the polymeric acid.
Capacitors are desired to have low equivalent series resistance (ESR), which is an index for impedance. To decrease ESR, conductivity of the solid electrolyte layer should be increased. Highly sophisticated control over conditions of the chemical oxidative polymerization is proposed to improve the conductivity of a solid electrolyte layer in Japanese Unexamined Patent Application, First Publication No. 11-74157. However, the proposed method may further complicate complicated and troublesome chemical oxidative polymerization, failing to simplify the process and reduce costs.
In some cases, π conjugated conductive polymers are used as an organic material which has a conductive mechanism of electronic conduction.
Resin films themselves are insulators and easily electrically charged. Furthermore, resin films tend to charge static electricity by friction or the like. Moreover, static electricity is not easily removed, but rather accumulates causing various problems.
Particularly, when resin films are used for wrapping food materials, in which sanitary considerations are emphasized, they may absorb dirt and dust and become largely deteriorated in appearance while on the shelves, which reduces the product's value. When resin films are used for wrapping powders, they absorb or repel powder that is charged while being wrapped or in use, making it inconvenient and difficult to handle the powder. When resin films are used for wrapping precision electronic devices, the precision electronic device may be broken due to static electricity. Therefore, steps must be taken to assure that static electricity does not occur.
Optical filters and optical information recording media are required that surfaces thereof are highly hard and transparent, and further comprise antistatic property for preventing adhesion of dust with static electricity. Particularly, the antistatic properties are required that a surface resistance value is stably within approximately 106 to 1010Ω (namely, stable antistatic properties). Therefore, an antistatic coating having antistatic and highly hard is provided on the surfaces of optical filters and optical information recording media.
For imparting antistatic properties, for example, a method that a resin film or surfactant is applied to the surface, or a method that a surfactant is mixed into a resin film or a resin composing an antistatic coating have been adopted (for example, see “Fine Chemical Antistatic Agents Latest Market Trend (the first volume),” Vol. 16, No. 15, 1987, p. 24-36, published by CMC).
However, since antistatic properties obtained by surfactants have a conductive mechanism of ion conduction, the surfactants are largely influenced by humidity, and, therefore, optical filters and optical information recording media are highly conductive at high humidity and poorly conductive at low humidity. Hence, the antistatic function is impaired and the antistatic ability is not exerted at need at low humidity and, particularly, under circumstances where static electricity easily occurs.
If metals and carbon having a conductive mechanism of electronic conduction are used, humidity-dependence is eliminated. However, since they do not have transparency, they are useless where transparency is required.
Metal oxides such as ITO (Indium Tin Oxide) have transparency and have a conductive mechanism of electronic conduction. Therefore, metal oxides are suitably used in transparency. However, if metal oxides are used, it is necessary to include a process using a sputtering apparatus or the like for making a film, so that processes are complicated and production costs become high. Furthermore, since inorganic metal oxide coatings have less flexibility, if the inorganic metal oxide is coated on a thin film base, the coating is extremely subject to cracks and does not exhibit conductivity. Moreover, since they have low adhesion property to an organic base, the inorganic metal oxide may be peeled off the organic base at the interface thereof and transparency may be decreased.
Known organic materials having a conductive mechanism of electronic conduction include π conjugated conductive polymers. Generally, π conjugated conductive polymers are insoluble and non-melting. Therefore, it is difficult to apply them to a film base after polymerization, and an attempt is made in which aniline is polymerized in the presence of a polymeric acid having a sulfo group to form a water-soluble polyaniline and the obtained mixture is applied to a film base and dried (for example, see Japanese Unexamined Patent Application, First Publication No. 1-254764).
An antistatic coating can be formed by directly polymerizing it on a base as in the method described in Japanese Unexamined Patent Application, First Publication No. 1-25476. In such a case, the antistatic coating has a low conductivity and shows poor adhesion to resin base due to water solubility, and further, the production process becomes more complicated.